In conventional heterodyne receivers the receive radio frequency (RF) is normally converted to one or more lower intermediate frequencies. The lower intermediate frequencies lend themselves more readily to the amplification and selective filtering required for processing received signals to derive information contained therein. The frequency conversion takes place in converter circuits wherein an input signal having an input frequency is converted to an output intermediate frequency (IF) signal by injecting a mixing signal at a frequency generated by a local oscillator (LO) into the input signal to mix the two signals together within a non-linear device, e.g., a diode. This non-linear mixing process produces various new signals at new frequencies. Examples of such new frequencies are: the sum of the input and LO frequencies, the difference of the input and LO frequencies, twice the sum of the input and LO frequencies, twice the difference of the input and LO frequencies, etc. These new frequencies are well understood by those skilled in the art. Of the various new signals generated, typically only the signal having a frequency equal to the difference of the input and LO frequencies is of interest, because it is the desired output IF signal.
In heterodyne receivers filtering is needed to attenuate unwanted products of the mixing process. One of these unwanted products is "second image." In a dual conversion receiver using "low side" injection and having a second IF of F2, the second image frequency would be 2.times.F2 below a received carrier frequency. In a dual conversion receiver using "high side" injection and having a second IF of F2 the second image frequency would be 2.times.F2 above a received carrier frequency. If the second image signal is not attenuated sufficiently by filters, the receiver converts the second image signal to a "base band" frequency that interferes with the desired output signal. The removal of the second image frequency by filter circuits following the IF converter circuits becomes more difficult as F2 becomes smaller, because as F2 becomes smaller, the second image frequency approaches the first IF, F1, which the filter circuit must pass with non-zero bandwidth.
In a battery powered heterodyne receiver, e.g. a portable selective call receiver, low power consumption is of critical importance. In active selectivity receivers it is possible to reduce power consumption by reducing the value of one or more intermediate frequencies. In the preceding example of a dual conversion receiver, power consumption can be reduced by reducing the second intermediate frequency F2. A limiting factor for the reduction of F2 is the ability of a filter circuit following the first IF converter to remove the undesirable second image frequency 2.times.F2 below the received carrier, for low side injection, and 2.times.F2 above the carrier, for high side injection.
Crystal filters normally are used following IF converter circuits, except for those converter circuits having low enough output frequencies to allow the use of solid state filter circuits. The performance of such crystal filters is such that for frequencies below the center frequency of the pass band of the filter, the attenuation of the filter is rapid, smooth, and monotonic. On the other hand, above the center frequency of the pass band of the filter, the attenuation of the filter is much less rapid, not smooth, and not monotonic, requiring approximately five times the incremental frequency change above the center frequency as that required below the center frequency to achieve reliably the same amount of attenuation. For this reason the lower second image frequency produced by low side injection is strongly preferred over the higher second image frequency produced by high side injection. Indeed, in some battery powered receivers having a low second IF it simply is impractical to use high side injection because of the associated filtering difficulties.
If the only consideration affecting the choice of high or low side injection were the best crystal filter performance, the decision would be simple: always use low side injection. Unfortunately, the choice of high or low side injection in conventional receivers is affected by another important phenomenon. That phenomenon is called "receiver self-quieting," a condition that occurs with certain combinations of LO frequencies, due to the mixing of selected harmonics of the LO frequencies. Receiver self-quieting occurs when the mixed LO harmonics result in second image frequencies that fall within the pass band of one or more IF circuits operating within the receiver. These spurious signals overpower a weak received signal, making the receiver insensitive at certain receive frequencies.
An example of a combination that will produce receiver self-quieting follows. Assume a receive frequency of 157.22 MHz, a first IF of 45 MHz, a second IF of 140 KHz, and the use of low side injection throughout. The first LO frequency is (157.22-45.0)=112.220 MHz, and the second LO frequency is (45.0-0.140)=44.860 MHz. The second harmonic of the first LO is 224.440 MHz, and the fifth harmonic of the second LO is 224.300 MHz. When mixed these two frequencies will produce an undesirable signal having a difference frequency of 140 KHz--identical to the second IF.
In conventional receivers it has been common practice to switch from low side injection to high side injection to overcome the problem of receiver self-quieting. Continuing with the preceding example, if the LO is moved to (157.22+45.0)=202.220 MHz (high side injection), then the self quieting problem goes away, because there are no low order harmonics of the two LOs that produce a difference frequency near the second IF.
In battery powered receivers having a low second IF and a crystal filter between first and second IF circuits, it may be impossible to sue high side injection for the reasons discussed previously. This leaves the designers of such receivers without a solution to the receiver self-quieting problem.
Thus, what is needed is a way to prevent receiver self-quieting without requiring the use of high side injection and its associated filtering difficulties.
One aspect of the present invention is a method for selecting an operating second intermediate frequency in a heterodyne receiver having a plurality of predetermined selectable receive frequencies and having first and second intermediate frequency (IF) circuits, the second IF circuit having a passband capable of being centered on at least two predetermined second intermediate frequencies, the first IF circuit being coupled to the second IF circuit for supplying an input signal thereto. The method comprises the steps of (a) selecting one of the plurality of predetermined selectable receive frequencies, and (b) selecting prior to generation of any undesirable spurious frequency one of the at least two predetermined second intermediate frequencies to be the operating second intermediate frequency for the second IF circuit in response to step (a). The operating second intermediate frequency is selected such that substantially all undesirable spurious frequencies generated will fall outside of the passband of the second IF circuit.
Another aspect of the present invention is a heterodyne receiver for receiving radio frequency signals comprising a receive frequency selector for selecting one of a plurality of predetermined selectable receive frequencies to be the operating receive frequency, and a controller coupled to the receive frequency selector for controlling the heterodyne receiver in response to the operating receive frequency selected. The heterodyne receiver further comprises a first intermediate frequency (IF) circuit coupled to the controller for generating a first intermediate frequency, and a second IF circuit coupled to the controller and to the first IF circuit and having a passband capable of being centered on at least two predetermined second intermediate frequencies. The controller comprises a second intermediate frequency selector for selecting prior to generation of any undesirable spurious frequency one of the at least two predetermined second intermediate frequencies to be the operating second intermediate frequency for the second IF circuit. The selected operating second intermediate frequency is such that substantially all undesirable spurious frequencies generated will fall outside of the passband of the second IF circuit.